Mounting evidence indicates that various human tumors contain populations of stem-like cancer cells, often termed cancer stem cells (CSCs) or tumor-initiating cells. Our lab, in the past few years, has focused on identification and functional characterizations of such tumorigenic cells in human prostate cancer (PCa). Our results have clearly demonstrated that PCa cells are organized as a tumorigenic hierarchy that contains subsets of relatively quiescent stemlike cancer cells, abundant and actively proliferating tumor progenitors, and the bulk differentiated PCa cells. Of significance, we have observed that the CD44+ PCa cell population contains both CSCs and tumor progenitors (i.e., PCa stem/progenitor cells). Recently, we have directed our efforts to understanding how PCa stem/progenitor cells are regulated and how regulatory molecules impact PCa development in vivo. In one project, for example, we have shown that Nanog, critical for ES cell selfrenewal and pluripotency, also plays an essential role in regulating PCa stem/progenitor cell properties and development of prostate (and other) cancers. In the current project, we show that a microRNA, miR-34a, plays a critical negative role in PCa stem/progenitor cells as well as PCa development. Importantly, preliminary data suggest that miR-34a exerts its negative impact on PCa stem/progenitor cells via targeting CD44 and Nanog. Based on our preliminary observations, we propose, in this R21 application, two Specific Aims: 1) to further study the role of miR-34a in regulating PCa stem/progenitor cells and tumor development;and 2) to test the hypothesis that CD44 and Nanog represent two critical downstream targets of miR-34a in PCa stem/progenitor cells. These aims will be accomplished by a spectrum of cell biological and molecular approaches combined with in vivo tumor experiments. The accomplishment of these goals will greatly advance our understanding of PCa stem/progenitor cell regulation and lay a solid foundation for developing novel anti-PCa therapeutics. PUBLIC HEALTH RELEVANCE: Prostate cancer (PCa) is the number one malignancy that inflicts and second leading cancer that kills American men. Increasing evidence suggests that PCa, like many other solid tumors, is fuelled by stem-like cells called cancer stem cells. In this project, we show that a microRNA, miR-34a, negatively regulates PCa stem/progenitor cells and potently inhibits PCa development. Importantly, miR-34a demonstrates potential therapeutic potential against PCa metastasis.